为 variardic 模板的每个参数调用模板函数

Question

我正在尝试使用专门的模板函数制作简单的 DI，如下所示：

#include <iostream>
template<typename T>
T inject();

template<>
int inject() {
    std::cout << "injected int" << std::endl;
    return 123;
}

class Test {
public:
    Test(int arg) {
        std::cout << arg << std::endl;
    }
};

template<typename C, typename... Args>
C* instantiate() {
    return new C(inject<Args>()...);
}

int main() {
    auto test = instantiate<Test>();
    return 0;
}

显然，它不起作用。是否可以像这样扣除可变参数 Args 并为每个变量调用 instantiate，将结果传递给给定类型 C 构造函数？

Answer 1

#include <type_traits>
#include <utility>
#include <cstddef>

struct injected {
    operator int() const { return 123; }
    operator double() const { return 3.14; }
    operator const char*() const { return "foo"; }
};

template <std::size_t, typename T>
using repeat = T;

template <typename C>
C* instantiate(std::index_sequence<0, 1, 2, 3, 4, 5>) {
    return nullptr;
}

template <typename C, std::size_t... Is>
auto instantiate(std::index_sequence<Is...>)
    -> std::enable_if_t<std::is_constructible<C, repeat<Is, injected>...>{}, C*> {
    return new C((void(Is), injected{})...);
}

template <typename C, std::size_t... Is>
auto instantiate(std::index_sequence<Is...>)
    -> std::enable_if_t<not std::is_constructible<C, repeat<Is, injected>...>{}, C*> {
    return instantiate<C>(std::index_sequence<Is..., sizeof...(Is)>{});
}

template <typename C>
C* instantiate() {
    return instantiate<C>(std::index_sequence<>{});
}

DEMO

为 variardic 模板的每个参数调用模板函数

Call template function for each argument of variardic template

c++

variadic-templates